There are several uncertainties there that will continue to puzzle us till the end of Trek...
Who exactly was going to stand down in six months? Kirk, Spock, McCoy and Scotty - the old geezers (none yet anywhere close to the 75 years established as retirement age in TAS "Counter-Clock Incident")? Or the whole bunch, including "young" Chekov? If the latter, it ought to be a matter of just disbanding the crew and sending these people to other assignments - it doesn't appear right that Chekov or Uhura should retire along with the elders. Or Spock, for that matter, being a long-lived Vulcan and all.
What exactly was Valeris going to continue doing when Spock stepped down? Being science officer to the
Enterprise? Being commanding officer to the
Enterprise, despite her low rank? Doing diplomacy with Klingons at the behest of Sarek? If one of the former, Spock must have been in the same belief as Kirk - that the ship would continue operating under a new crew for a few years at least.
Did Chekov or Scotty retire? In ST:GEN, everybody is still in uniform, but Kirk is assuredly and explicitly retired, so there's no telling.
If Captain Kirk, following TFF gets got one more Five Year Mission, it still have been over well before Praxis exploded. Sulu would have left during it to command Excelsior. However if instead Starfleet had gone to using Three Year Missions, like what Excelsior is just finishing, Enterprise could have had one or even two of those since TFF.
Amusingly, we have no evidence that anybody else would ever have conducted a "five year mission" but Kirk in TOS. And it's not as if this is even a valid a mission type in TOS, something our heroes know started on stardate X and will end on stardate Y: it seems our heroes simply keep on doing stuff both at the far frontier and at home waters, and end up doing five years of it before recalled for refitting of their ship or whatever. (It's in ST:ID that Kirk knows in advance that he will be spending exactly five years out there in unexplored space - something that did
not happen in TOS.)
Quite possibly Starfleet just keeps all ships on "active reserve", sending them to whatever assignment is called for, and every once in a while recalls them for refit and total rotation of crew. At that point, it starts referring back to the active period as an "X-year mission" (at least in the intro to the popular holoshow based on the adventures), even though basically none of it was preplanned or otherwise what we would think a "mission" entails.
Seeing how the Federation Council handed Kirk this new Enterprise, it seems like he is permanently attached to her. No other captain is given the commanding officer's position of Enterprise as an assignment. Just Kirk. So wherever Enterprise is to go, Kirk will be on the bridge.
And, apparently, vice versa: when Kirk personally is needed to sort out the Nimbus III crisis, the dysfunctional
Enterprise is sent with him. Was that, too, a case of Nixon in China - of Starfleet trying to intimidate the Klingons with a big-name hardliner, or, conversely, to placate the voters back home with a hardliner even though he actually had orders to behave?
Timo Saloniemi